1048 THE VASCULAK SYSTEM. 



Iliac Arteries and their Branches. The common iliac arteries are formed from the 

 secondary roots of the umbilical arteries, and their exact morphological position is uncertain. 

 The true morphological position of the hypogastric arteries is not yet denned. They also are 

 parts of the secondary roots of the umbilical arteries, and they give off both somatic and 

 splanchnic branches ; therefore they do not correspond either with somatic intersegmental or 

 with splanchnic arteries. The branches of the hypogastric artery are arranged in two groups 

 (1) a visceral set which supplies the walls of the hind-gut and the genital organs, and (2) a 

 parietal set which is distributed to the body wall and to the hind-limbs. The branches dis- 

 tributed to the gut probably represent the splanchnic vessels, more or less homologous with 

 ordinary splanchnic branches of the primitive aortse, and the parietal branches are possibly 

 the homologues of intersegmental arteries. 



THE LIMB ARTERIES. 



In all probability the vessels of both the upper and the lower limbs are derived originally 

 from several somatic intersegmental arteries, the majority of which, however, have atrophied. 

 The upper limb is supplied in man by the lateral offset from the ventral branch of the seventh 

 somatic intersegmental artery. It passes into the extremity caudal to the shoulder girdle, courses 

 through the arm, enters the cubital fossa, and is continued through the forearm, in the early 

 stages, as the volar interosseous artery, which terminates in the deep part of the palm, in the 

 deep volar arch. At a later period, ontogenetically, a median artery appears as a branch of 

 the parent stem, and it terminates in a superficial volar arch ; still later the radial and ulnar 

 branches are formed. The latter grow rapidly, soon exceeding in size the parent stem, and they 

 terminate in the superficial and deep volar arches. The interosseous and median arteries decrease, 

 and generally lose their direct connexions with the volar arches. The dorsal interosseous artery 

 is also a secondary branch from the parent stem, and the digital arteries are offsets from the volar 

 arterial arches. 



The chief arteries of the lower extremities spring directly from the secondary roots of the 

 umbilical arteries, and may be looked upon as being essentially intersegmental ; whether they 

 represent the whole or only parts of typical somatic intersegmental arteries, however, is not clear. 



The arteries of the lower limbs certainly show no very obvious indications of division into 

 dorsal and ventral branches, though such indications are not entirely wanting. In their com- 

 parative absence it is supposed that the dorsal branches have been either suppressed or incor- 

 porated with the common stems ; that similarly the ventral branches and their lateral offsets 

 are indistinguishably fused, and that probably both are represented in a limb artery. 



The original stem vessel of the lower limb is the inferior glutaeal artery, which is continued 

 distally, caudal to the pelvic girdle, into the popliteal and peroneal arteries, and so to the 

 plantar arch. Subsequently the external iliac artery is given off from the secondary root of the 

 umbilical artery, dorsal to the origin of the inferior glutseal, and, passing into the limb cranial- 

 ward of the pelvic girdle, it becomes the femoral artery. This vessel ultimately unites with 

 the proximal part of the popliteal artery, and after this communication is established the distal 

 part of the inferior glutaeal atrophies and loses its connexion with the popliteal, which hence- 

 forth appears to be the direct continuation of the femoral trunk ; therefore, whilst the main 

 artery of the upper limb is formed by the prolongation of the lateral branch of one segments,! 

 artery, the corresponding vessel of the lower extremity is developed from representatives of, 

 probably, two somatic segmental arteries, the external iliac and femoral trunks being the 

 representatives of one, whilst the popliteal and its continuation, the peroneal, are parts of 

 another. 



The first main artery of the leg, ontogenetically, is the peroneal, which is continued into 

 the plantar arch ; after a time, however, the posterior and anterior tibial branches are given 

 off from the stem, over which, as a rule, they soon preponderate in size, and they terminate in the 

 plantar arch, whilst the parent trunk diminishes and loses its direct connexion with the arch. 



The peroneal artery corresponds in position and development with the common interosseous 

 trunk and the volar interosseous artery in the forearm. The posterior tibial apparently corre- 

 sponds with the median artery; it develops in a similar way, and has similar relations to 

 homologous nerves, the tibial nerve representing the combined median and ulnar nerves 

 of the upper extremity. 



The anterior tibial artery represents the dorsal interosseous, whilst the radial and ulnar 

 arteries of the upper extremity are not represented in the lower limb. 



MORPHOLOGY OF THE VEINS. 



Two dorsal longitudinal vessels, one on each side, connect the successive intersegmental 

 veins together. They do not, however, in any part of their course, fuse together to form a 

 single vessel comparable to the descending aorta. 



Of these dorsal longitudinal vessels, that on the right side greatly enlarges, and from it t 

 main stem vessels which return blood from the body walls, the head and neck, and the limbs, are 

 almost entirely formed. The left dorsal longitudinal vessel remains relatively small in parts, 

 indeed, it altogether disappears and the blood conveyed to it by the corresponding intei 

 segmental veins is transmitted, across the median plane, to the chief functional stem by later 



